The present invention relates to spunmelt nonwovens, and more particularly such spunmelt nonwovens which are hydroengorged.
Spunmelt nonwovens (e.g., spunbond or meltblown nonwovens) are formed of thermoplastic continuous fibers such as polypropylene (PP), polyethylene terephthalate (PET) etc., bi-component or multi-component fibers, as well as mixtures of such spunmelt fibers with rayon, cotton and cellulosic pulp fibers, etc. Conventionally, the spunmelt nonwovens are thermally, ultrasonically, chemically (e.g., by latex), or resin bonded, etc., so as to produce bonds which are substantially non-frangible and retain their identity through post-bonding processing and conversion. Thermal and ultrasonic bonding produce permanent fusion bonds, while chemical bonding may or may not produce permanent bonding. Typically fusion-bonded spunmelt nonwovens have a percentage bond area of 10-35%, preferably 12-26%.
Generally, the prior art teaches that hydroentanglement of a spunmelt nonwoven requires that, in order to increase or maintain tensile strength, the spunmelt nonwoven initially be essentially devoid of fusion bonds and that any bonds initially present be of the frangible type which are to a large degree broken during the hydroentanglement process. See, for example, U.S. Pat. Nos. 6,430,788 and 6,321,425; and U.S. Patent Application Publication Nos. 2004/0010894; and 2002/0168910. Hydroentanglement of such unbonded or frangibly bonded spunmelts is used primarily to add integrity and therefore tensile strength to the spunmelt nonwoven.
In order to facilitate conversion (that is, further processing of a spunmelt nonwoven), it is necessary that the nonwoven have an appropriate tensile strength for the conversion processing. The acceptable “window” for tensile strength will vary with the intended conversion processing.
In the case of the unbonded or frangibly bonded spunmelt nonwovens, the initial integrity or tensile strength is very low, and the use of a hydroentanglement step increases the integrity and tensile strength (relative to what it was before) such that the spunmelt nonwoven can undergo the conversion process. However, the prior art generally teaches that, because of the nature of the fusion bonded spunmelt nonwoven prior to hydroentanglement, such spunmelt nonwovens subsequent to hydroentanglement exhibit only a limited level of integrity and a relatively low tensile strength, one which is frequently substantially diminished, relative to the tensile strength of the fusion bonded spunmelt nonwoven prior to hydroentanglement, due to breakage of the fibers. Thus, hydroentanglement of fusion bonded spunmelt nonwovens may lower the integrity and tensile strength of the spunmelt nonwoven to such an extent that it is no longer suitable for the desired subsequent conversion processing.
Accordingly, it is an object of the present invention to provide, in one preferred embodiment, a hydroengorged spunmelt nonwoven formed of thermoplastic continuous fibers and a pattern of fusion bonds.
Another object is to provide, in one preferred embodiment, such a spunmelt having a percentage fusion bond area of less than 10%.
A further object is to provide, in one preferred embodiment, such a spunmelt nonwoven having a percentage fusion bond area of at least 10% wherein the pattern of fusion bonds is anisotropic.
It is also an object of the present invention to provide, in one preferred embodiment, such a spunmelt nonwoven which exhibits after hydroengorgement an increase in caliper of at least 50% and a tensile strength of at least 75% of the tensile strength exhibited by the spunmelt nonwoven prior to hydroengorgement.